An electric power steering apparatus (EPS) that applies an assist torque (an assist force) with a rotatory power force of a motor to a steering system of a vehicle, gives a driving force of the motor as an assist to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate a torque of a steering assist force, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of a PWM (Pulse Width Modulation) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. A column shaft (a steering shaft or a steering wheel shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting the steering torque Th of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command based on the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a voltage control command value Vref obtained by performing compensation and so on with respect to the calculated current command value. A steering angle sensor 14 is not absolutely necessary, does not need to be disposed, and further a steering angle can be obtained through a rotational angle sensor such as a resolver or the like connected to the motor 20.
A controller area network (CAN) 50 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN 50. Further, a Non-CAN 51 is also possible to connect to the control unit 30, and the Non-CAN 51 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 50.
The control unit 30 mainly comprises a CPU (including an MPU and an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 (or from CAN 50) are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 that is a control target value that is supplied to the motor 20 by using an assist map and so on based on the inputted steering torque Th and the vehicle speed Vel.
The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A, a current command value Irefm of which a maximum current is limited is inputted into a subtracting section 32B, a deviation I (=Irefm−Im) between the current command value Iref3 and a motor current value Im that is fed back is calculated, and the deviation I is inputted into a PI (proportional and integral)-control section 35 that is a current control section for improving an characteristic of a steering action. The voltage control command value Vref of which a characteristic is improved by the PI-control section 35 is inputted into a PWM-control section 36, and the motor 20 is PWM-driven through an inverter 37 as a driving section. The current value Im of the motor 20 is detected by a motor current detector 38, and is fed back to the subtracting section 32B. FETs are used as driving elements within the inverter 37, which comprises a bridge circuit of FETs.
Further, in a case that the motor 20 is a brushless motor, since it is necessary to current-apply a current to a motor coil depending on a rotational angle of the motor, a rotational sensor 21 that uses a resolver or a magneto-resistive element (MR sensor) is needed. An output signal of the rotational sensor 21 is inputted into an angle detecting circuit 22 and then is processed. The angle detecting circuit 22 detects a rotational angle θ, and further an angular velocity calculating section 23 calculates an angular velocity ω.
A compensation signal CM from a compensation signal generating section 34 is added to an adding section 32A, and a characteristic compensation of the a steering assist system is performed by adding the compensation signal CM, and improves a convergence, an inertia characteristics and so on. The compensation signal generating section 34 adds a self-aligning torque (SAT) 343 and an inertia 343 at an adding section 344, adds the convergence 341 to an addition result at an adding section 345, and makes an addition result at the adding section 345 to be the compensation signal CM.
In the case that a high operability and a reliability of such an electric power steering apparatus are required, a high precision and a high failure-detectability of a rotational angle detecting mean are demanded. Therefore, 2-systematization (dual system) of the rotational angle detecting means and digitalizing (since SENT (Single Edge Nibble Transmission) standard and so on have a strong noise immunity) of a signal transmission means from a sensor module to an ECU have been strongly required.
For example, a brushless motor controlling method which the reliability is improved due to a redundancy is disclosed in Japanese Published Unexamined Patent Application No. 2004-194490 (Patent Document 1). That is, the apparatus thereof comprises plural Hall sensors, and there are provided the first sensor group that detect a rotor rotational position by detecting a magnetism of the rotor magnet and the second sensor group that comprise plural Hall sensors which are arranged with a space by a deviation angle to the Hall sensors. Then, the apparatus performs an overlap current-applying control by using the both sensor groups in the case the Hall sensors work normally, and performs a square-wave control by using the sensor group that does not contain a failed sensor in the case the Hall sensor is failed.